These cars came with power antenna as an option.
Most 5th Ave’s had power antennas – so not hard to find working OE (Original Equipment) parts to retrofit your car with.
A tip, if you find any early ‘70’s and up Chrysler product with power antenna, and the antenna mast is up position – then it has a bad power antenna motor or stripped out mast cord (or it was un-plugged in the up position for some really odd reason). In other words - keep looking.
Attached (below) is some pages from my ’86 FSM (Factory Service Manual) that might be helpful, whichever route you chose (should be very close to your ’81).
I highly recommend to either get the official antenna nut tool (can be obtained on eBay or other places – and looks like tool used below)
OR what I use is a larger set of 90 degree snap ring pliers.
You “can” use a screwdriver or channel-lock priers – but either will tear things up (especially a screwdriver vs. your paint job).
BudW